Chemical Changes (paper 1)

What happens when metals react with oxygen?

• Metal oxides are produced.

• Oxidation reaction

What is an oxidation reaction?

• A reaction in which oxygen is gained

What is a reduction reaction?

• When oxygen is lost.

How do we measure the reactivity of magnesium,zinc,iron and copper as they don’t react with water at room temperature ?

• React them with dilute acids

• Don’t react group 1 metals with dilute acids as it’ll react dangerously fast.

The reactivity series

• Potassium

• Sodium

• Lithium

• Calcium

• Magnesium

• Carbon

• Zinc

• Iron

• Hydrogen

• Copper

• Silver

• Gold

What determines how rapidly a metal reacts?

• When metals react, they lose electrons and form a positive ion.

• E.g. K→ K+ +e-

• The reactivity of a metal depends on its ability to lose electrons and form a positive ions.

• Metals at the top of the series can lose electrons and form a positive ion more easily than metals at the bottom.

How does potassium react with water?

• burns with a lilac flame

• effervescence because of production of hydrogen

• production of salt

How does sodium react with water?

• burns with a flame

• effervescence because of production of hydrogen

• production of salt

• sodium disappears- dissolves

How does lithium react with water?

• burns with a flame

• effervescence because of production of hydrogen

• production of salt

• lithium eventually disappears

How does calcium react with water?

• effervescence because of production of hydrogen

• production of salt

• reacts slower than g1 tho

How does magnesium react with water?

• steam

• some fizzing

• quite slow

How does zinc react with water?

How does iron react with water?

How does copper react with water?

How does potassium react with dilute acid?

How does sodium react with dilute acid?

How does lithium react with dilute acid?

How does calcium react with dilute acid?

How does magnesium react with dilute acid?

How does zinc react with dilute acid?

How does iron react with dilute acid?

How does copper react with dilute acid?

(Room temp)

How are unreactive metals found?

• As the metal (by) itself

• An example of this is gold

• Contrastingly, iron and copper react with oxygen to form compounds.

How can we extract the metal that we want in a metal compound?

• Get a more reactive element in the reactivity series

• However because most of these are expensive we mainly use carbon as it’s cheap.

Oxidation and reduction in terms of electrons

• Oxidation is the loss of electrons

• Reduction is the loss of electrons

• OIL RIG

• e.g. Mg→ Mg 2+ + 2e-

• This is a half equation showing magnesium has been oxidised.

• e.g. S+2e- → S2-

• Sulfur has been reduced

Formula of hydrochloric acid

• HCl (aq)

• (aq) means aqueous solution- dissolved in water.

• In aqueous solutions, acids produce H+ ions

• Example:

• H2SO4 (aq)→ 2H+ (aq) + SO4 2- (aq)

Formula of sulfuric acid

• H2SO4 (aq)

Nitric acid

• HNO3 (aq)

What is a base?

• Chemicals which can neutralise acids, producing a salt and water.

• Bases are usually metal oxides or metal hydroxide.

Examples of bases

• Copper oxide

• Iron (III) hydroxide

• Sodium hydroxide

• Sodium hydroxide is soluble in water, making it an alkali and a base whereas the other two are only bases because they aren’t soluble in water.

What are alkalis?

• Bases which are soluble in water.

• In aqueous solutions, alkalis produce hydroxide ions (OH-)

• Example: NaOH (aq) → Na+ (aq) + OH- (aq)

What do acids have a pH between?

• 0-6

What are solutions with a pH of 7?

• Neutral

What do alkaline solutions have a pH between?

• 8-14

How can we determine the pH of a solution?

• Using a pH probe

• Universal indicator

What does a pH probe do?

Determines the pH of a solution electronically.

What does universal indicator do?

Changes colour depending on whether a solution is acid alkali or neutral.

Deceive colours on the pH scale

• Red= very acidic

• Green= neutral

• Very purple= alkaline

Neutralisation reaction between acid and alkali?

• Acids produce H+

• Alkalis produce OH-

• H+(aq) +OH- (aq)→ H2O (l)

• When we react an acid with an alkali the hydrogen ions react with the hydroxide ions to produce water (which is neutral).

What do all acids contain?

Hydrogen

Which metals can this place hydrogen from acids?

• Potassium to iron

• This is because they’re more reactive

What is produced when acids react with metals?

Salt and hydrogen gas

How do we know which salt we’ll produce?

• Depends on the acid and metal

What’s the ending of the salt produced when hydrochloric acid reacts with a metal?

Chloride

What’s the ending of the salt produced when sulfuric acid reacts with a metal?

Sulphate

What do acids produce in aqueous solutions?

• Hydrogen ions

• H+

2HCl + Mg → MgCl2 + H2

• Mg → Mg 2+ +2e-

• It has been oxidised

• 2H+ +2e- → H2

• It’s been reduced as it’s gaining electrons

• This is a redox reaction

What is a redox reaction?

• A reaction in which both oxidation and reduction is happening.

Redox reaction

Ca + FeSO4 → CaSO4 + Fe

• Write with charges

• Form ionic equation.

• Get rid of spectator ion from ionic equation.

• Form half equation and see what’s been oxidised and what’s been reduced

When an acid is reacted with either a base or an alkali, what is produced?

• A salt and water

What do salts contain?

• A positive ion which comes from the base or alkali

• Salt also contains negative ion which comes from the acid.

Metal carbonates

• Sodium carbonate Na2CO3

• Calcium carbonate CaCO3

• Potassium carbonate K2CO3

When acids react with a metal carbonate, what is made?

• Salt, water and carbon dioxide

• E.g.

• Hydrochloric acid + Sodium Carbonate→ Sodium Chloride + water + carbon dioxide

Required practical: Making soluble salts

• Get equipment: beaker, gauze, tripod, heatproof mat and Bunsen burner.

• Gently heat up 20 cm³ of sulfuric acid until almost boiling.

• Use a spatula to add small amounts of copper oxide to the acid and then stir the solution with a glass rod.

• The copper oxide will react and dissolve in the acid to form a blue solution.

• Then add excess copper oxide (until there’s remaining unreacted powder in the solution) to solution in order to ensure that all of the sulfuric acid has reacted (to prevent it from contaminating the salt).

• Get filter funnel and filter paper so you can carry out filtration in order to remove the unreacted copper oxide.

• Then to get the salt from the solution, put the solution into an evaporating dish and heat it gently till half of the water has evaporated from the salt.

• Then leave for 24 hours in a cool place so it can crystalise to form crystals- if you evaporate all the water immediately then a white powdered salt will be formed.

What do acid molecules do in aqueous solutions?

• They ionise and release H+

Strong and Weak acids

• Strong acids fully ionise in aqueous solutions, most or all of the acid molecules release their H+ ions.

  • arrows go in one direction only showing that the acid has fully ionised

• Weak acids partially ionise in aqueous solutions.

  • arrows go in both directions, showing it’s a reversible reaction so not all of the acid molecules have ionised (only some).

Examples of strong acids

• HCl

• H2SO4

• HNO3

Examples of weak acids

• Carbonic acid

• Ethanoic acid- vinegar

• Citric acid

The pH scale

• Strong acids have a lower pH then weak acids for a given concentration.

• As the pH scale decreases by one unit, the concentration of H+ ions increases by 10 times, meaning that it’s 10 times more acidic.

What do H+ ions determine?

• How strong an acid is

Dilute and concentrated acids

• This is different from the strength of the acid.

• Dilute acid will have fewer acid molecules in a given volume than a concentrated acid even if the strength of the acid is the same.

Required Practical: Titrations

• Use a volumetric pipette to transfer 25 cm³ of sodium hydroxide solution into a conical flask.

• Transfer the alkali onto a white tile- this is so the colour change can be observed more clearly.

• Add a few drops of indicator such as methyl orange or phenolphthalein to the conical flask.

• Now fill a burette with sulfuric acid and record the starting volume.

• Add acid to the alkali whilst swirling the conical flask to ensure that the acid and the alkali are mixing.

• To do this accurately add the acid drop by drop when a colour change is seen.

• Do this until the solution is neutral.

• Read the volume of acid added from the burette at the bottom of the meniscus making sure your eyes is level with the surface of the liquid.

• Repeat the process until you get at least two concordant results.

• Use the concordant results to calculate an average titre.

• Use the titre volume to calculate the unknown concentration of the acid.

  • This is for concentration of acid- switch the acid and alkali if you want concentration of alkali.

What are titrations used for?

• To calculate the concentration of either an unknown alkali or acid when we know the volume and concentration of the other